1. Field of the Invention
The present invention relates to a method of detection of a welding workpiece position which uses the movable electrode to detect a surface position of a welding workpiece in a spot welding system which uses a multiarticulated robot to make the welding workpiece and a spot welding gun move relative to each other, clamps the welding workpiece between a facing movable electrode and counter electrode of the spot welding gun, and spot welds the welding workpiece.
2. Description of the Related Art
A spot welding system is provided with a spot welding gun having a movable electrode driven by a servo motor and a counter electrode arranged facing the movable electrode and with a multiarticulated robot holding the spot welding gun at its front end. It uses the multiarticulated robot to make the welding workpiece and spot welding gun move relative to each other, then closes the movable electrode and counter electrode of the spot welding gun toward a predetermined strike position on the welding workpiece to clamp the welding workpiece between the movable electrode and counter electrode of the spot welding gun and, in this state, applies voltage across the two electrodes to thereby perform spot welding at the strike position. The counter electrode is generally a fixed electrode provided on a gun arm. In such a spot welding system, to enable the movable electrode and counter electrode to be positioned at the strike position on the welding workpiece, the surface position of the welding workpiece at the movable electrode side and surface position at the counter electrode side at the strike position have to be taught to the multiarticulated robot in advance.
In the teaching work, the worker visually confirms the position of the welding workpiece and operates the multiarticulated robot to make the spot welding gun move to a position where the movable electrode and counter electrode contact at the strike position of the welding workpiece when closing the movable electrode and counter electrode of the spot welding gun. After making the spot welding gun move to the desired position, the position of the multiarticulated robot and position of the movable side electrode tip are stored. In other words, positions based on the position of the welding workpiece are taught for the position of the multiarticulated robot and position of the movable side electrode tip.
In the above such teaching work, the positions have to be taught so that the movable electrode and counter electrode can suitably contact the surface of the welding workpiece. If such a judgment of contact of the movable electrode and counter electrode with the welding workpiece is performed visually by a worker, depending on the degree of skill of the worker and the viewing conditions in the work environment, contact may not be able to be accurately detected and therefore detection of the surface position of the welding workpiece at the movable electrode side and surface position at the counter electrode side at the strike position also becomes uncertain. Therefore, for example, as described in Japanese Patent No. 3337448, the method has been proposed of utilizing the increase in the value of a current of a servo motor (that is, a value of a torque) caused by action of a reaction force caused by bending, denting, or other elastic deformation of a welding workpiece when a movable electrode is moved under the drive force of a servo motor toward a counter electrode and contacts the surface of the welding workpiece and judging that the movable electrode and welding workpiece have contacted each other when the value of the current of a servo motor reaches a predetermined threshold value and of utilizing the generation of a reaction force to the multiarticulated robot from the welding workpiece caused by elastic deformation of the welding workpiece when a counter electrode is moved by the multiarticulated robot toward the welding workpiece and contacts the surface of the welding workpiece and judging that the counter electrode and welding workpiece have contacted each other when the reaction force to the multiarticulated robot reaches a predetermined value. Note that, the reaction force to the multiarticulated robot is found from the values of the current of the servo motors driving the different axes of the multiarticulated robot. Further, as described in Japanese Patent Publication (A) No. 6-218554, there is also the method of using the same method as in Japanese Patent No. 3337448 to detect contact of the movable electrode and welding workpiece, then using the multiarticulated robot to move the spot welding gun so as to make the counter electrode approach toward the welding workpiece until the distance between the movable electrode and counter electrode become equal to a thickness of the welding workpiece at a strike position measured in advance, using the servo motor to make the movable electrode move toward the counter electrode by the same speed of movement as the spot welding gun so as to maintain the state of the movable electrode contacting the welding workpiece, and thereby making the counter electrode and welding workpiece contact and position the counter electrode on the welding workpiece.
Further, Japanese Patent No. 4233584 proposes a method using the same method as in Japanese Patent No. 3337448 to detect contact between the movable electrode and welding workpiece, then making the counter electrode move toward the welding workpiece by exactly the value of the difference between the distance between the movable electrode and counter electrode at that time and a set thickness of the welding workpiece and thereby positioning the counter electrode at that position.
To shorten the time required for detection of the surface position of a welding workpiece using a movable electrode in teaching work of a spot welding system, it is necessary to make the movable electrode move faster. If making the movable electrode move, the dynamic friction of the drive mechanism of the movable electrode causes the current or torque of the servo motor to vary (fluctuate). Therefore, if making the movable electrode move at a fast speed, the variation of the current or torque of the servo motor also becomes greater. As a result, due to the variation of the current or torque of the servo motor caused by the dynamic friction of the drive mechanism of the movable electrode, the current or torque of the servo motor ends up exceeding a threshold value for judging contact and erroneous detection of contact of the movable electrode and welding workpiece occurs in some cases.
To prevent this erroneous detection, it is necessary to set the threshold value of the current or torque of the servo motor for judging contact of the movable electrode and welding workpiece to a value larger than the variation of the current or torque of the servo motor due to dynamic friction. However, if the threshold value is large, the movable electrode is moved from when actually contacting the welding workpiece to when it is judged to have contacted the welding workpiece, so the amount of elastic deformation of the welding workpiece becomes larger and the surface position of the welding workpiece is no longer able to be accurately detected. Further, in the worst case, the welding workpiece is liable to end up being caused to plastically deform.
Furthermore, the amount of elastic deformation of the welding workpiece becomes the sum of the “push-in distance” over which the movable electrode is made to move from when actually contacting the welding workpiece to when judged to contact the welding workpiece and the “runaway distance” from when it is judged that the welding workpiece has been contacted to when the movable electrode tip actually stops. However, the runaway distance can be calculated from the deceleration time of operation of the movable electrode tip, while the push-in distance depends on the material or fastening method of the welding workpiece and the mechanical rigidity of the spot welding gun, so it is necessary to consider the deformation model of a complex workpiece. This cannot be found by simple calculation. Therefore, it is not possible to strictly calculate the amount of elastic deformation of the welding workpiece.
Further, the increase in the current or torque of the servo motor used when detecting that the movable electrode has contacted the welding workpiece depends on the reaction force from the welding workpiece to the movable electrode due to the elastic deformation of the welding workpiece, so in the case of a soft welding workpiece, even with the same amount of elastic deformation of the welding workpiece, the reaction force becomes smaller than with a hard welding workpiece and, as a result, the amount of change of the current or torque of the servo motor when the movable electrode and welding workpiece contact becomes smaller. Therefore, in the case of a soft welding workpiece, the change of the current or torque of the servo motor when the movable electrode and welding workpiece contact is hidden by the variation of the current or torque of the servo motor due to the dynamic friction, so detection of the contact of the movable electrode and welding workpiece becomes difficult and the precision of detection of contact deteriorates.
On the contrary, if slowing the speed of movement of the movable electrode, the fluctuation or variation of the current or torque of the servo motor due to the dynamic friction of the movable electrode drive mechanism can be made smaller and further, as a result, the threshold value can be set to a small value, so the above two problems can be solved. However, if slowing the speed of movement of the movable electrode, the problem arises that the time required for detection of the surface position of the welding workpiece by the movable electrode becomes longer and therefore the operation becomes impractical.
Accordingly, a first object of the present invention is to eliminate the above problems of the prior art and provide a spot welding system in which the precision of detection the surface position of a welding workpiece by a movable electrode can be improved without lengthening the time required for detection of the surface position of the welding workpiece by the movable electrode.
In the method described in Japanese Patent Publication (A) No. 6-218554, to position the counter electrode at the surface of the welding workpiece, it is necessary to preset the thickness of the welding workpiece at each strike position. However, setting the thickness of the welding workpiece takes time, so there is the problem that it is difficult to set the thicknesses at each of the strike positions, which may number in the hundreds, in actual welding work. Furthermore, if setting the thickness of the welding workpiece at each strike position based on the values in design data of the welding workpiece, due to the presence of gaps between welding workpieces and individual differences in welding workpieces, the counter electrode sometimes does not contact the welding workpiece, the counter electrode sometimes is excessively pressed against the welding workpiece causing the welding workpiece to deform, and, in the worst case, the welding workpiece is made to plastically deform and the spot welding gun is damaged. Therefore, there is the problem that to prevent deformation of the welding workpiece or damage to the spot welding gun, before setting the thickness of the welding workpiece, it is necessary to measure in advance the thickness of the welding workpiece at all strike positions on the welding workpiece.
On the contrary, with the method described in Japanese Patent No. 3337448, it is not necessary, like in the method of Japanese Patent Publication (A) No. 6-218554, to measure the thickness of the welding workpiece at all strike positions. However, the reaction force from the welding workpiece to the multiarticulated robot, used for detecting the contact of the counter electrode with the welding workpiece, is due to the elastic deformation of the welding workpiece arising when the counter electrode contacts the welding workpiece. Therefore, for the reaction force to exceed a threshold value, the welding workpiece has to be made to deform by a certain extent. The threshold value depends on the rigidity of the welding workpiece. Further, the changes in the values of the currents of the servo motors driving the axes of the multiarticulated robot due to the reaction force from the welding workpiece to the multiarticulated robot depend, in addition to the rigidity of the welding workpiece, on the rigidity of the arm of the spot welding gun and the multiarticulated robot. Therefore, the threshold value of the reaction force used for detecting the contact of the counter electrode with the welding workpiece differs depending on the combination of the welding workpiece, spot welding gun, and multiarticulated robot used. To accurately detect the contact of the counter electrode with the welding workpiece, it is necessary to find the threshold value of the reaction force in advance experimentally, then set it. For this reason, there is the problem that it is difficult to handle a broad range of workpieces.
Accordingly, a second object of the present invention is to eliminate the above problems in the prior art and provide a spot welding system in which it is possible to accurately detect the surface position of a welding workpiece to which a counter electrode contacts without regard as to the rigidity of the welding workpiece, spot welding gun, and multiarticulated robot.
In the methods disclosed in Japanese Patent No. 3337448 and Japanese Patent No. 4233584, when detecting the surface position of the welding workpiece at the movable electrode side, when using a servo motor to drive the movable electrode to make it move so as to make it approach toward the counter electrode, the value of the current or the value of the torque of the servo motor driving the movable electrode is monitored. However, when the movable electrode is moving driven by a servo motor, due to the mechanical resistance present at the spot welding gun (for example, internal friction of the drive part, elastic deformation of the conductive parts connecting the movable electrode and the welding transformer, etc.), variation or fluctuation of the current or torque of the servo motor driving the movable electrode occurs and an unstable state is easily caused. For this reason, the increase in the current or torque of the servo motor occurring when the movable electrode contacts the welding workpiece ends up being hidden by the variation or fluctuation of the mechanical resistance of the spot welding gun and the point of time of contact of the movable electrode and welding workpiece becomes difficult to accurately identify or erroneous detection of contact of the movable electrode and welding workpiece occurs.
Accordingly, a third object of the present invention is to solve the above problems in the prior art and provide a spot welding system improving the precision of detection of the surface position of a welding workpiece by a movable electrode.